1. Field of the Invention
The present invention relates to current-switching circuits of the type used, for example, in fluorescent lighting control systems for selectively connecting one or more electronic ballasts to an alternating-current (AC) power source.
2. Description of the Related Art
Typically, gas discharge lamps, such as fluorescent lamps, must be driven by ballasts (such as electronic dimming ballasts) in order to illuminate. A common control method for dimming ballasts is “zero-to-ten-volt” (0-10V) control (which is sometimes referred to as 1-10V control). A 0-10V electronic dimming ballast receives power from an AC power source, with an external mechanical switch typically coupled between the AC power source and the 0-10V ballast to provide switched-hot voltage to the ballast. The 0-10V ballast controls the intensity of the connected lamp in response to a 0-10V control signal received from an external 0-10V control device. Often, the 0-10V control device is mounted in an electrical wallbox and comprises an intensity adjustment actuator, e.g., a slider control. The 0-10V control device regulates the direct-current (DC) voltage level of the 0-10V control signal provided to the ballast between a substantially low voltage (i.e., zero to one volt) to a maximum voltage (i.e., approximately ten volts) in response to an actuation of the intensity adjustment actuator.
When applying power to the electronic ballast, the ballast behaves as a capacitive load. Thus, when the mechanical switch is closed to turn on the fluorescent lamp, there is a large in-rush of current into the ballast, which quickly subsides as the ballast charges up to line voltage. This temporary current surge can be problematic as the number of electronic ballasts controlled by a mechanical switch increases. For example, in the case of a full 16-amp (steady-state) circuit of dimming ballasts, the in-rush current can approach 560 amps. Though short-lived, e.g., only a few line cycles or shorter, this level of surge can wreak havoc on the contacts of even a relatively large relay having a high current rating (e.g. 50 amps). The problem stems from the fact that each time a pair of contacts of the mechanical switch close or snap together, there is a tendency for the contacts to bounce apart. When this bouncing occurs during a large current surge, the intervening gas or air ionizes and arcing occurs. The arcing has the effect of blasting away the conductive coatings on the relay contacts which eventually causes the relay to fail, either due to erosion of the contact material, or, more commonly, due to welding of the contacts in the closed position.
Accordingly, prior art lighting control systems including 0-10V ballasts have required heavy-duty mechanical switches, which tend to be physically large and costly. Such mechanical switches are too large to fit in a single electrical wallbox and thus must be mounted in a separate enclosure than the 0-10V control device. An example of a prior art 0-10V control device that requires an externally-mounted relay is the Nova T-Star® 0-10V Control, model number NTFTV, manufactured by Lutron Electronics Co., Inc.
Other prior art switching circuits for ballasts have required advanced components and structures (such as microcontrollers and multiple relays per ballast circuit), and complex wiring topologies (such as requiring a neutral connection). An example of such a switching circuit is described in greater detail in commonly-assigned U.S. Pat. No. 5,309,068, issued May 3, 1994, entitled TWO RELAY SWITCHING CIRCUIT FOR FLUORESCENT LIGHTING CONTROLLER, and U.S. Pat. No. 5,633,540, issued May 27, 1999, entitled SURGE-RESISTANT RELAY SWITCHING CIRCUIT. The entire disclosures of both patents are hereby incorporated by reference.
Therefore, there is a need for a simple analog 0-10V load control device that fits in a single electrical wallbox and provides both the switched hot voltage and the 0-10V control signal to a 0-10V ballast. Further, there is a need for a simple two-wire switching circuit that can handle a large inrush current, but that does not require a neutral connection or a heavy-duty mechanical switch or relay.